In some instances, cellular senescence is thought to contribute to the development and/or progression of age-related disease, but in others, the presence of disease may accelerate the accumulation of senescent cells.

A recent study has provided strong evidence to suggest that intervertebral disc degeneration, a major cause of low back pain, is due to accelerated cellular senescence (Le Maitre et al, 2007). Cells isolated from normal and degenerate human tissue were assessed for mean telomere length, SA-β-Gal, and replicative potential. Mean telomere length decreased with age in cells from non-degenerate tissue and also decreased with progressive stages of degeneration. SA-β-Gal staining was not observed in non-degenerative patients unlike cells from degenerative discs which did exhibit 10-12% SA-β-Gal staining and decrease in replicative potential. However, the factors which may have led to accelerated senescence in this instance was not discussed. There are three possible reasons why cellular senescence was accelerated in this instance; (1) Unknown factors resulted in the damage and removal of cells, resulting in cell turnover for replacement, (2) ROS were involved causing stress induced premature senescence (SIPS) or (3) telomeres in these cells for some unknown reason started off shorter than normal, meaning less cell turnover is required for the appearance of senescent cells.

Other studies have shown a correlation between disease states and the presence of senescent cells in vivo. SA-β-Gal staining was used to detect senescent cells in normal liver, liver with chronic hepatitis C and hepatocellular carcinoma (HCC) (Paradis et al, 2001). They found senescent cells present in 3 of 15 (20%) normal livers tested, 16 of 32 (50%) in livers with chronic hepatitis C and in 6 of 10 (60%) livers with HCC. The presence of senescent cells in normal livers was found to be associated with old age. Interestingly, the presence of senescent cells in non-tumoural tissues was strongly correlated with the presence of HCC in the surrounding liver. This demonstrates not only that the ageing of one tissue can have a direct impact on another but also as suggested by Judith Campisi, senescent cells may contribute to carcinogenesis (Campisi, 1997).

Another study looked at cellular senescence in human benign prostatic hyperplasia (BPH) specimens (Choi et al, 2000). BPH is a disease associated with an abnormal growth of the adult prostate that begins mid to late life. Results from this study found that 40% of the analysed samples showed positive staining for SA-β-Gal and only in the epithelial cells. A high prostate weight (> 55g) was found to correlate strongly with the expression of SA-β-Gal. Prostates weighing less than 55g tended to lack senescent epithelial cells. It was suggested that the accumulation of senescent epithelial cells may play a role in the development of prostatic enlargement associated with BPH. However, the accumulation of senescent cells in this case is likely to be a consequence of the disease, which may lead further to its progression. The enlargement of the prostate may be the result of unregulated stimulated proliferation, increasing cell turnover and consequently the appearance of senescent cells. This may explain why a stronger expression of SA-β-Gal is detected in prostates weighing more than 55g since they may have undergone more cellular divisions.

During the pathogenesis of type 2 diabetes, insulin resistance causes compensatory proliferation of pancreatic beta cells. This compensatory proliferation might accelerate cellular senescence contributing further to the progression of diabetes. To investigate this, one group used nutrient-induced diabetic mice to analyse beta cells for SA-β-Gal and the proliferation marker Ki67 (Sone and Kagawa, 2005). At 4 months, the proliferation of beta cells was 2.2 fold higher than in the control group. At 12 months, the frequency of Ki67 decreased to one-third that of the control and SA-β-Gal positive cells increased to 4.7 fold that of the control group. This increase in the senescent beta cell fraction correlated with insufficient insulin release, suggesting cellular senescence may contribute to diet-induced diabetes. In this instance it is difficult to determine whether senescence is the cause or the consequence of insulin resistance. It later appears to be a contributor but whether it is also the initiating factor is unknown.

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An increase in the number of senescent primary lung fibroblasts has also been shown to increase in patients with emphysema compared with normal controls (Muller et al, 2006). An average of 4% of cells from control patients stained positive for SA-β-Gal compared to an average of 16% in patients with emphysema. It is possible that long-term exposure to tobacco smoke accelerates the formation of senescent cells, which subsequently may lead to loss of elasticity of the lung tissue, destruction of structures supporting the alveoli, and destruction of capillaries feeding the alveoli observed with emphysema. One study has shown for example that cigarette smoke induces senescence in alveolar epithelial cells (Tsuji et al, 2004).

Another study used senescent associated p16 instead of SA-β-Gal to detect senescent cells in kidneys with glomerular disease (GD) (Sis et al, 2007). Glomerular diseases include many conditions which fall into two major categories; Glomerulonephritis describes the inflammation of the membrane tissue in the kidney that serves as a filter, separating wastes and extra fluid from the blood. Glomerulosclerosis is the scarring or hardening of the tiny blood vessels within the kidney. This study found an increased expression of the nuclear p16 in samples with GD compared with normal. Independently, older age and interstitial inflammation was associated with increased expression of nuclear p16. Since senescent cells adopt a pro-inflammatory phenotype, their presence may be a contributing factor in inflammation observed in glomerulonephritis.

All these examples demonstrate the presence of senescent cells not only in vivo, but more specifically in disease states. This suggests that the accumulation of senescent cells in normal tissues is the result of injuries to those tissues or in some cases unregulated stimulation of proliferation. This suggests that if no injuries occurred as a consequence of disease, environmental factors or by normal biological/mechanical wear and tear, ageing of mitotic tissues would be greatly reduced.

Whether cellular senescence is the cause or the consequence of some diseases is yet to be answered.

The early observation that young cultured fibroblasts have a higher growth potential than those derived from adults led to the proposal that senescent cells may play a causal role in organismal ageing (Hayflick, 1961). For this to be the case, senescent cells need not only to be shown to be present in living tissue but also to persist for long periods. However, cellular senescence at the time of Hayflick’s proposal was thought by many to be a tissue culture artefact, with no relevance to normal human ageing. Since the growth conditions of cultured cells are dissimilar to that found in vivo, it was thought that these differences resulted in the formation of senescent cells in culture but not in vivo. It has also been argued that if senescent cells are present within tissues, the fraction would be so small that they are unlikely to have any impact on the surrounding tissue and ageing in general.

At the time of Hayflicks proposal there was no way of detecting senescent cells in vivo. It wasn’t until over 30 years later that a marker was used to demonstrate the presence of senescent cells in human dermis in vivo (Dimri et al, 1995). This detection system is based on a modified beta-galactosidase assay and is termed senescence-associated beta-galactosidase (SA-β-Gal). Skin samples were taken from 20 human donors aged 20-90 years, sectioned and stained for SA-β-Gal. Results showed an age-dependent increase in SA-β-Gal staining in dermal fibroblasts and epidermal keratinocytes. None of the young donors (<40yr)>69 yr) donor did display positive staining. About half of the young donors showed some epidermal staining whereas positive staining was always observed in the epidermis of old donors. Using the same technique to identify senescence cells, another group looked at senescence of the retinal pigment epithelium (RPE) of Rhesus monkeys (Hjelmeland et al, 1999). Results also showed an accumulation of SA-β-Gal positive cells in the eyes of older monkeys. Another study also found little or no SA-β-Gal staining in HCECs of corneas from young donors but was easily detected in corneas from older donors (Mimura and Joyce, 2006). .

More recent studies have used other markers to detect cellular senescence in mitotic tissues (Herbig et al, 2006, Jeyapalan et al, 2007). These studies investigated cellular senescence in the tissues of ageing primates. They used markers of senescence such as telomere damage, active checkpoint kinase ATM, high levels of heterochromatin proteins and elevated levels of p16 in skin biopsies from baboons with advancing age. The number of dermal fibroblasts containing damaged telomeres reached a value of over 15% of total fibroblasts in very old animals (26-30 years) compared to young (5-6 years) where DNA damage were rarely detected. However, in skeletal muscle, a postmitotic tissue, only a small percentage of damage to telomeres was detected regardless of age.

In contrast to the TDS and TIS mechanisms, cell cycle independent senescence (CCIS) does not seem to require cell division. A number of signals have been shown to trigger CCIS, including oxidative stress (Von Zglinicki et al, 2000), histone deacetylase inhibitors (Munro et al, 2004), and expression of some activated oncogenes such as ras, raf and MEK (Di Micco et al, 2006). This section focuses on oxidative stress and activated oncogenes.

The proposal that oxidative stress may cause CCIS comes from studies that have treated cells with concentrations of H2O2 and found that those cells enter a long-term growth arrest resembling replicative senescence; often called stress induced premature senescence or SIPS. One such study treated human fibroblasts with sub-lethal concentrations of hydrogen peroxide which induced cell cycle arrest, with an initial 2-3 fold increase in the level of p53 protein and subsequently an increase in the level of p21 protein (Chen et al, 1998). One study has shown that the up-regulation of Caveolin-1 by oxidative stress is required to induce premature senescence (Volonte et al, 2002). Caveolin-1 is thought to function as a “transformation suppressor” protein. For example Caveolin-1 mRNA and protein expression are lost or reduced during cell transformations by activated oncogenes.

Oxidative stress is thought to result in damage to DNA and DNA mutations. The most severe type of DNA damage is double-strand breaks (DSBs). Studies have shown that cells treated with agents that cause DSBs lead to an increase in p16 expression and premature senescence (Robles and Adami, 1998). Induction of DNA damage resulted in the induction of p53 and p21. The concentration of p21 protein increases 100-fold and then begins to drop, followed by an increase in p16. This is similar to what occurs during replicative senescence.

Oxidative stress is also thought to result in DNA becoming mutated and this may impact oncogenes such as RAS. Studies have shown that mutated oncogenic Ras can result in a permanent G1 arrest (Serrano et al, 1997). Oncogenic Ras is commonly associated with the transformation of primary cells to an immortal state. It seems, however, that this transformation can only occur with either the co-operation of another oncogene or the inactivation of tumour suppressers such as p53 or p16. Serrano and co-workers also found that Ras induced cell senescence is accompanied by the accumulation of p53 and p16 and that the inactivation of either one of these prevents Ras induced arrest in rodent cells. More recent studies have also shown that Ras can induce cell senescence in vascular smooth muscle cells (Minamino et al, 2003) and endothelial cells (Spyridopoulos et al, 2002). In the first study, an activated ras allele was introduced into human vascular smooth muscle cells using retroviral infection. This resulted in growth arrest with phenotypic characteristics of cell senescence. In the second study, bovine aortic endothelial cells were infected using the adenovirus containing the activated ras gene. Over-expression of ras in this case led to G1 and G2/M-cell cycle arrest after 72 hours due to induction of p21. p21 induction again appears to be the initiating factor of Ras induced senescence and induction of p16 is required to maintain the senescent state. One group for example found that Ras was capable of causing growth arrest in both p21 and p53 negative human fibroblasts, suggesting Ras is activating the p16 pathway (Wei et al, 2001).

An overview of telomere-dependent, telomere-dependent and cell-cycle independent senescence is shown below.

Like p21, p16 is a CDK inhibitor involved in cell cycle arrest. Over expression of p16 causes G1 arrest in early passage cells by inhibiting the phosphorylation of the retinoblastoma protein (Kato et al, 1998). However, the removal of p16 activity results in only minimal lifespan extension that was terminated by senescence (Wei et al, 2003). p16 plays a role in maintaining senescence triggered by a short telomere. It appears that p21 is the initiating factor in the growth arrest observed in senescent cells, but p16 is required to maintain that state. p21 has been shown to initially increase in fibroblast cultures but later gradually decrease (Alcorta et al, 1996). During the decline of p21, p16 protein levels gradually increased in senescent cultures, reaching nearly 40-fold higher than in early cultures.

P16 is thought to trigger senescence independently of telomere length. For example, the inactivated p53 in two different fibroblast strains: WI-38 (from foetal lung) and BJ (from neonatal foreskin) resulted in the continuation of growth without Rb inactivation in BJ cells, whereas WI-38 cells fail to grow even when both Rb and p53 are inactivated. It was suggested that these two outcomes was due to the intrinsic differences in the ability to induce p16 at senescence. This proposal was based on the understanding that like WI-38, other human cells appear to undergo replicative senescence prior to telomere shortening by the induction of p16 (Itahana et al, 2003). The ability of p16 to cause growth arrest even after Rb inactivation also suggests that p16 can function independently of Rb activity. Further support for a telomere-independent mechanism was provided from studies which have shown that telomerase activity alone was insufficient to extend the replicative potential of human keratinocytes or mammary epithelial cells (Kiyono et al, 1998). It was shown that down-regulation of p16 in combination with telomerase activity did lead to replicative lifespan extension. Another study found that cyclin D1 over-expression in primary oral keratinocytes extend the lifespan, whereas the combination of cyclin D1 over-expression and p53 inactivation led to their immortalisation (Opitz et al, 2001). Cyclin D1 forms complexes with CDK4 which subsequently phosphorylates and inactivates retinoblastoma (Rb) growth repression (Connell-Crowley et al, 1997). P16 inhibits cell cycle progression by inhibiting CDK4. It appears that the over-expression of cyclin D1 bypasses p16-TIS resulting in an extension of lifespan. In this instance the p16 mechanism has become redundant and the cells continue to divide until TDS mechanism is activated. This may be why these cells are immortalised by p53 inactivation.

This telomere-independent mechanism may be the primary trigger of cellular senescence in mice and rats. Mice and rats have telomeres which are 5-10 times longer than those of human cells (Shay and Wright, 2001). Despite this increase in length, rodent cells engineered to lack telomerase show telomere shortening with no effect on replicative potential. These cells senesce in the presence of long telomeres. Since mouse and rat cells repair DNA damage far less efficiently than do human cells, it has been suggested that such damage may be the trigger for p16 induction.

Telomere attrition is thought to be the predominant mechanism which ultimately leads to a cell becoming senescent, caused by the inability of polymerase to completely replicate DNA at the 5’ end. The presence of a short telomere is thought to disrupt a protective telomeric structure, exposing loose ends of DNA which trigger a DNA damage response and consequently causes a cell to enter cellular senescence.

Human telomeres consist of short tandem GC-rich repeats (TTAGGG/AATCCC). These telomeric repeats are predominantly double-stranded with a 3’ single-stranded overhang terminating with a G-rich tail (Collins, 2000). The proposed mechanism by which telomeres and their associated proteins protect the chromosome ends is by creating large terminal loops known as a t-loop (Greider, 1999). In this structure, the double-stranded telomeric DNA is looped around and the 3’ single-strand overhang invades the duplex of telomeric repeats and forms a displacement loop (D-loop) structure is important for maintaining chromosomal stability. Without this protection, it is thought that the ends of chromosomes would become exposed, resembling double-strand breaks which can trigger cell-cycle arrest and senescence (Shore, 2001).

Telomeres can be elongated by adding telomeric repeats to the 3’ end of DNA. This can be carried out by telomerase, an enzyme which consists of an RNA molecule and a catalytic component known as hTERT. Telomerase is a reverse transcriptase which uses its RNA component as a template to reverse transcribes DNA back to the ends of chromosomes. Telomerase activity is repressed in most somatic tissue and reactivated in ~90% of human cancers (Artandi, 2006). Introduction of telomerase into normal somatic cells has been shown to extend replicative life-span (Bodnar et al, 1998) and not induce changes associated with a malignant phenotype (Jiang et al, 1999). Since telomerase is not present in somatic cells, they progressively get shorter.

The majority of short-telomere growth arrested cells is thought to occur in the G1 phase of the cell cycle, a process regulated by proteins such as p21. The appearance of DNA damage results in the activation of p53, a transcription factor whose activation results numerous downstream responses including the up-regulation of p21 and activation of DNA repair proteins. P21 is a cyclin dependent kinase (CDK) inhibitor which halts cell cycle progression by directly interacting and inhibiting cyclin-CDK complexes and consequently preventing the phosphorylation of Rb.

Experiments in which p21 was inactivated within senescent cells resulted in the ability of these cells to re-enter the S phase of the cell cycle, but were still unable to divide (Ma et al, 1999). It was concluded that p21 plays an important role in the maintenance of senescence and in the inhibition of S-phase progression, but inhibition of p21 is insufficient to permit cells to complete the cell cycle. Since p21 expression has been shown to be dependent upon p53, the impact of p53 inactivation on cell cycle progression was also investigated (Bond et al, 1999). Fibroblasts lacking p53 function bypassed replicative senescence and continued to proliferate for another 20 CPD before entering growth arrest. The point at which growth arrest occurs due to replicative senescence is also known as M1, whereas the second growth arrest point if replicative senescence is bypassed is known as M2 or crisis. Cells which enter crisis result in apoptosis rather than senescence. This suggests that when a cell becomes senescent at M1 it may not be due to the disruption of the telomere-end structure. Since there appears to be a reserved replicative capacity, it may only be at M2 that the telomere end-structure becomes disrupted, and this may be the reason why apoptosis is common at this point. The idea that a cell becomes senescent long before the telomere-end structure is disrupted suggests that telomere length may not be the trigger for replicative senescence and instead is due to another mechanism.

One such mechanism may be alterations or exposure of the telomere 3’ single-stranded overhang. One study reported that exposure of telomere 3’ overhang sequences induces senescence (Li et al, 2003). In this study, cells were exposed to oligonucleotides homologous to the telomere 3’ overhang sequence TTAGGG for 1 week. As a result, a senescent phenotype in cultured fibroblasts was observed. The team concluded that exposure the 3’ overhang by t-loop disruption or possibly DNA damage leads replicative senescence. Telomere shortening may result in the exposure of the 3’ overhangs, but the telomere end-structure is still maintained efficiently to prevent complete disruption. Interestingly, this 3’ overhang has been found to be eroded at replicative senescence indicating it as a possible trigger (Stewart et al, 2003). It was indicated that overhang erosion is the result of continuous cell division and not a consequence of senescence. Therefore it was concluded that this alteration in telomere structure and not overall telomere length serves as a trigger for replicative senescence.